The present invention relates to an electric motor-powered drive device for reducing the revolutions of an electric motor and transmitting the reduced revolutions to an output shaft through an electromagnetic clutch.
In more particularly, the present invention relates to a motor-powered drive device for use as, for example, a drive device for driving a throttle valve of an internal combustion engine to transmit a drive force of a motor through a reduction mechanism and an electromagnetic clutch to an output shaft of the motor-powered drive device.
FIG. 1 is a cross section of a conventional electric motor powered drive device and FIG. 2 is a cross section taken along a line II--II in FIG. 1. In these figures, an electric motor 1 has an output shaft on which a pinion 2 is provided. A stepped gear 3 includes a large gear 3a meshed with the pinion 2 and a small gear 3b meshed with an output gear 5. The pinion 2, the stepped gear 3 and the output gear 5 constitute a reduction gear mechanism. A clutch rotor 6 has an outer periphery formed with the output gear 5 and rotatably supported on an output shaft 10 through a bearing 10a. A movable clutch plate 7 faces toward the rotor 6 and is provided on a side end of a flange 9 fixed to the output shaft 10 through leaf return springs 8. A clutch yoke 11 rotatably supports the output shaft 10 through a bearing 10b and forms a magnetic circuit of an electromagnetic clutch 13. A coil 12 is wound on the yoke 11. A rotary lever 14 is fixed to an end of the output shaft 10 by a nut 15 and has a groove 14a in which a wire 16 for driving an external load is wound. A return spring 17 is provided for biasing the rotary lever 14 in clockwise direction when viewed in the direction of the arrow in FIG. 1. A potentiometer 18 is coupled with the output shaft 10 and provides a resistance value corresponding to a rotation angle thereof. Reference numerals 19 and 20 denote housing portions of the drive device and 21a and 21b are stoppers for restricting rotation of the rotary lever 14.
The operation of the above-noted device will now be described. When a voltage is applied to the motor 1, it rotates and a rotation force is transmitted through the pinion 2 and the stepped gear 3 to the output gear 5. Under such a condition, when there is no current flowing through the coil 12 of the electromagnetic clutch 13, the clutch rotor 6 rotates freely with respect to the output shaft 10, so that no rotational driving force is transmitted to the output shaft 10. When current is supplied through the coil 12, the movable clutch plate 7 is magnetically attracted by the rotor 6, so that the rotational force of the rotor 6 is transmitted through the movable clutch plate 7 and the flange 9 to the output shaft 10 to rotate the latter. A control circuit (not shown) responds to a rotation angle (desired value) of the rotary lever 14 to drive the motor 1 to thereby rotate the output shaft 10 through the electromagnetic clutch 13. The rotation angle of the output shaft 10 can be detected as a variation of the resistance value of the potentiometer 18 coupled with the output shaft 10. Further, since the control circuit drives the motor 1 until the resistance value (control amount) of the potentiometer 18 coincides with the desired value, a desired rotation angle of the rotary lever 14 can be obtained.
Since the conventional motor-powered drive device is constructed as mentioned above, grease lubricant on the output gear 5 formed on the clutch rotor 6 tends to be scattered to the magnetic attracting plane of the rotor 6, so that the friction force is reduced and the transmission torque of the electromagnetic clutch 13 is substantially reduced.
Further, the magnetic flux produced by the current flowing through the coil 12 leaks to the clutch rotor 6, the output shaft 10 and the flange 9 which are formed of magnetic material. As a result, the effective flux passing through the attracting plane between the clutch rotor 6 and the movable clutch 7 is reduced, resulting in a reduction of the transmission torque of the electromagnetic clutch.
In such a motor-powered drive device, in order to shorten the time necessary to rotate the rotary lever 14 to the desired rotation angle, a possibility of over-rotation of the rotary lever 14 tends to increase. In the conventional motor-powered drive device, in which the movable clutch plate 7 is coupled through the leaf return springs 8 to the clutch plate 7 of the electromagnetic clutch 13 and a compression force is exerted on the return springs 8 when the clutch rotor 6 is driven in the clockwise direction when viewed from the left side in FIG. 1, the compression force becomes large when the rotary lever 14 is prevented from further rotation by the stopper portion, resulting in a buckling of the rotary lever 14.
Further, the drive torque of the motor 1 acts to compress the return springs 8 when an over-rotation of the rotary lever 14 is restricted by the stopper portion 21a during clockwise rotation. As a result, the return spring 8 is buckled.